1. Field of the Invention
The present invention relates to a reception device, a control method, and a program and, more specifically, to a reception device that receives a modulation signal being the result of digital modulation of a carrier with various modulation techniques, and a control method and a program that can enhance the capabilities of the reception device.
2. Description of the Related Art
With BS (Broadcasting Satellite) digital broadcasting, a modulation technique of subjecting a carrier to digital modulation includes BPSK (Binary PSK (Phase Shift Keying), QPSK (Quadrature PSK), and 8PSK, which are available for a dynamic selection.
It means that, with BS digital broadcasting, transmitted is a modulation signal being the result of modulating a carrier by any of BPSK, QPSK, and 8PSK. The modulation is performed based on any target data for transmission.
With BS digital broadcasting, data transmission is made on a super frame basis.
FIG. 1 shows the format of a modulation signal of a super frame to be received by a reception device, which receives BS digital broadcast services.
The super frame is configured by eight frames of #1, #2, #3, #4, #5, #6, #7, and #8.
The frame #i (i=1, 2, . . . , and 8) is configured by symbol sets being sequentially in order from its head. The symbol sets include a set of 32 symbols of unique data W1, a set of 128 symbols of TMCC (Transmission Multiplexing Configuration Control), a set of 32 symbols of unique data W2 or W3, and 4×48 sets of 203 symbols of a main signal and 4 symbols of a burst signal. As such, the frame #i is configured by 39936 (=32+128+32+(203+4)×4×48) symbols.
The symbols of the data W1, W2, and W3 are each unique and known, and subjected to BPSK modulation. The symbols are used for establishing frame synchronization in the reception device.
Note that, in the frame #i, the symbols of the data W1 and the symbols of the data W2 or W3 are so disposed as to sandwich the symbols of TMCC therebetween. Specifically, among the eight frames of #1 to #8 configuring the super frame, the frame #1 includes the symbols of the data W1 and the symbols of the data W2, and the remaining frames of #2 to #8 each include the symbols of the data W1 and the symbols of the data W3.
In the below, as appropriate, the symbols of the data W1, W2, and W3 are respectively referred to as unique symbols W1, W2, and W3, and either the unique symbols W2 or those of W3 are referred to as unique symbols W2/W3 as appropriate.
The TMCC is control information including a modulation technique, a coding technique, and others for the main signal in the modulation signal. The symbols of the TMCC are subjected to BPSK modulation.
With modulation including digital modulation, a carrier is modulated in accordance with any target data for transmission so that a modulation signal is derived. In this specification, for convenience of description, modulating a carrier in accordance with any target data for transmission is referred also to as modulating any target data for transmission.
The symbols of the TMCC can be decoded on a frame basis, and in the reception device, the resulting TMCC after decoding of the symbols thereof is used to find the modulation technique for the main signal in the modulation signal so that the main signal can be demodulated.
Note that, in the reception device, the TMCC of a super frame is subjected to Reed-Solomon decoding so that the modulation technique can be found for the main signal in the resulting modulation signal.
The main signal is original information exemplified by coding data or others that are supposed to be transmitted. The coding data here is the result of applying MPEG coding to image data. The symbols of the main signal are modulated by any of the modulation techniques of BPSK, QPSK, and 8PSK.
Note that the modulation technique for the symbols of the main signal can be defined on a symbol type basis, i.e., the symbols of the burst signal, or the 203 symbols of the main signal sandwiched between the unique symbols W2/W3 and the symbols of the burst signal.
The burst signal is a PRBS (Pseudo Random Bit Sequence)(Pseudo Random Binary Sequence) signal of a frame period, and the symbols of the burst signal are those known to be subjected to BPSK modulation.
Herein, the symbols of the burst signal are referred to as burst symbols as appropriate.
The burst symbols are provided intermittently in the modulation signal to enable establishment of carrier synchronization in the reception device even if with a low C/N (Carrier to Noise Ratio).
That is, in the reception device, through multiplication of a carrier, a modulation signal is demodulated to a demodulation signal including an I component of the same phase as that of the carrier and a Q component orthogonal to the carrier. The issue here is that there is generally an error between a carrier for use by the reception device and a carrier for use by a transmission device from which the modulation signal comes. As a result of such an error, the symbols of the demodulation signal derived in the reception device rotate on an IQ plane, which is defined by an I axis indicating the I component and a Q axis indicating the Q component.
In the reception device, for the aim of compensating such rotation of the symbols of the demodulation signal, the carrier synchronization is established.
Specifically, in the reception device, a phase error is detected, and the phase error is filtered by a loop filter, for example. The phase error is of between a signal of predetermined phase coming from an NCO (Numerically Controlled Oscillator) and symbols of the demodulation signal. The filtering result by the loop filter is then used as a basis to control the NCO, and based on the phase error, phase rotation is performed with respect to the symbols of the demodulation signal in such a manner as to correct the phase error.
As such, for establishing carrier synchronization, when symbols are being subjected to QPSK or 8PSK modulation with which a distance between signal points is relatively small on the IQ plane, if a C/N is low, it may cause a difficulty in establishing carrier synchronization.
In consideration thereof, as shown in FIG. 1, the modulation signal includes intermittently the burst symbols that are known and have been subjected to BPSK modulation with which a distance between signal points is large. With such burst symbols, even if a C/N is low, carrier synchronization can be established in the reception device.
Patent Document 1 (Japanese Patent No. 3205313) describes a technology of changing the gain of a loop filter for use to establish carrier synchronization in accordance with a ratio of average error between a phase direction of a demodulation signal and an amplitude direction thereof.
Patent Document 2 (Japanese Patent No. 3646010) describes about the setting of, based on the phase noise characteristics of an antenna, a filter coefficient of a loop filter for use to establish carrier synchronization.